Why can't I make item of clothing in the MakeHuman model?
Here's a imported MHX character from Makehuman.



It is already rigged, so the mesh of the skin (which is just a surface) is somehow controlled by several handles.

There are a some ways in which a mesh can be rigged. In this case the character was built with an Armature.
We can see in fact that the skin object (see the picture above for reference) has an Armature Modifier pointing to the armature (the top object of the hiererchy).

Every movement of the armature will be followed by a movement of the skin as defined by the armature's properties (bone weights, vertex groups, etc...).

When you switch from Object mode to Edit mode, the modifiers by default are not shown, so the skin doesn't follow the armature's deformation anymore. The mesh can be probably found near the origin in resting position.



You can edit the mesh "in place" by enabling the modifier visibiity for edit mode and adjusting the cage (see enabled options near the cursor)